The ever-increasing world population and the dwindling supply of arable land available for agriculture fuels research towards developing plants with increased biomass and yield. Conventional means for crop and horticultural improvements utilize selective breeding techniques to identify plants having desirable characteristics. However, such selective breeding techniques have several drawbacks, namely that these techniques are typically labor intensive and result in plants that often contain heterogeneous genetic components that may not always result in the desirable trait being passed on from parent plants. Advances in molecular biology provide means to modify the germplasm of plants. Genetic engineering of plants entails the isolation and manipulation of genetic material (typically in the form of DNA or RNA) and the subsequent introduction of that genetic material into a plant. Such technology has the capacity to deliver crops or plants having various improved economic, agronomic or horticultural traits.
Traits of interest include plant biomass and yield. Yield is normally defined as the measurable produce of economic value from a crop. This may be defined in terms of quantity and/or quality. Yield is directly dependent on many factors, for example, the number and size of the organs, plant architecture (for example, the number of branches), seed production, leaf senescence, photosynthetic carbon assimilation and more. Root development, nutrient uptake, stress tolerance and early vigor may also be important factors in determining yield. Optimizing the abovementioned factors may therefore contribute to increasing crop yield.
An increase in seed yield is a particularly important trait since the seeds of many plants are important for human and animal consumption. Crops such as corn, rice, wheat, canola and soybean account for over half the total human caloric intake, whether through direct consumption of the seeds themselves or through consumption of meat products raised on processed seeds. They are also a source of sugars, oils and many kinds of metabolites used in industrial processes. Seeds contain an embryo (the source of new shoots and roots) and an endosperm (the source of nutrients for embryo growth during germination and during early growth of seedlings). The development of a seed involves many genes, and requires the transfer of metabolites from the roots, leaves and stems into the growing seed. The endosperm, in particular, assimilates the metabolic precursors of carbohydrates, oils and proteins and synthesizes them into storage macromolecules to fill out the grain. An increase in plant biomass is important for forage crops like alfalfa, silage corn and hay.
Plants are often characterized by their method of photosynthesis, with most plants using C3, C4, or CAM photosynthesis. While there are a number of plant species that appear to be capable of utilizing one type of photosynthesis under certain environmental conditions and another type of photosynthesis under different environmental conditions, most plants primarily use one of these three classes of photosynthesis. A number of highly productive and economically important crop plants including maize, sugarcane, sorghum, millet, switchgrass, and Miscanthus sp. use C4 photosynthesis. Additionally, a major research effort is ongoing to convert rice (Oryza sativa) from a C3 to a C4 plant by using the tools of genetic engineering (available online at the website for the C4 Rice Project). C4 plants are characterized by their cellular specialization, with a ‘C4 carbon shuttle’ used as a way to concentrate CO2 in bundle sheath cells after it diffuses into mesophyll cells from the atmosphere. Typically, in C4 plants, CO2 is first converted into oxaloacetate (C4H2O42−) in mesophyll cells; the four carbons in oxaloacetate give the C4 photosynthetic pathway its name. This oxaloacetate then undergoes a series of chemical reactions and is transported into bundle sheath cells where it is fixed via the Calvin-Benson cycle into molecules that the plant uses for its growth. C4 plants are more productive than C3 plants in some environments, but methods to improve the productivity of C4 plants are desired.
As described above, crop yield is a trait that is controlled by many factors. One contributing factor is the rate of photosynthetic carbon assimilation by the plant. By increasing the rate of carbon assimilation, plant growth and ultimately plant yield may be increased. Therefore, methods for increasing photosynthetic carbon assimilation, particularly C4 photosynthetic carbon assimilation, are desired.